This invention relates to a wideband optical amplifier and a wideband variable wavelength optical source. More particularly, this invention relates to a wideband optical amplifier that can amplify optical signals with wavelengths ranging from 1.55 xcexcm band (C-band: 1.53-1.565 xcexcm) to 1.58 xcexcm band (L-band: 1.565-1.60 xcexcm) and a wideband variable wavelength optical source using such an optical amplifier.
Optical communication systems and devices using optical fiber cables require wideband optical amplifiers and optical sources. FIG. 1 shows an example of such a wideband optical amplifier in the conventional technology. This example is a wideband optical amplifier for amplifying optical signals ranging from C-band to L-band. The more details of which is shown in Japanese Patent Laid Open No. Hei 10-229238 and xe2x80x9cElectron Letter, 33, pp 710, 1997, M. Yamada et. al.xe2x80x9d This conventional example is briefly explained here with reference to FIG. 1.
As shown in the block diagram of FIG. 1, the wideband optical amplifier is mainly comprised of a C-band optical amplifier 100, an L-band optical amplifier 200, an optical demultiplexer and an optical multiplexer. The wideband optical amplifier receives an input optical signal 10s and produces an output optical signal 62s by amplifying the input optical signal 10s. 
The C-band optical amplifier 100 includes a first optical isolator 11, a first erbium doped optical fiber (EDF) 21, a first pump light source 31, a WDM (Wavelength Division Multiplexing) coupler 31c, and a second optical isolator 12. The L-band optical amplifier 200 includes a third optical isolator 13, a second pump light source 32, a WDM coupler 32c, a second erbium doped optical fiber (EDF) 22, a third pump light 33, a WDM coupler 33c, and a fourth optical isolator 14. In this example, the optical demultiplexer and optical multiplexer are a WDM coupler 61 and a WDM coupler 62, respectively.
The input optical signal 10s provided to the WDM coupler (demultiplexer) 61 is divided into optical signals 10s1, and 10s2. The optical signal 10s, is supplied to the first optical isolator 11 in the C-band optical amplifier 100 and the optical signal 10s2 is supplied to the third optical isolator 13 in the L-band optical amplifier. Instead of the WDM coupler 61, other type of optical demultiplexer or an optical switch may be used.
In the C-band optical amplifier, the first optical isolator 11 blocks the light moving in the opposite direction, i.e., backward scattering lights, and provides the input optical signal 11s to the first erbium doped optical fiber 21. Thus, by the first optical isolator 11, unwanted lights, such as pumping lights in a backward direction are blocked from travelling toward the input side.
The first erbium doped optical fiber 21 is used as an amplifying medium and has a fiber length optimized to amplify signals in the C-band. For instance, the first erbium doped optical fiber 21 has a fiber length of 20 m (meter) . The first erbium doped optical fiber 21 receives a pump light from the first pump light source 31 through the WDM coupler 31c. Based on a laser operation in the rare earth element (erbium) doped fiber, the first erbium doped optical fiber 21 amplifiers the input signal 11s by several ten dB, such as 20 dB or more, to produce an amplified optical signal 21s. The second optical isolator 12 receives the amplified optical signal and produces an optical signal 12s at its output. The second optical isolator 12 blocks lights propagating in the backward direction.
As noted above, the first pump light source 31 and the WDM coupler 31c provide the pump light to excite the first erbium doped optical fiber 21. In this example, the pump light source 31 is placed at the back side of the first erbium doped optical fiber 21 so that the pump light travels in the backward direction (backward pumping).
In the L-band optical amplifier 200, the third optical isolator 13 blocks the light moving in the opposite direction, i.e., backward scattering lights, and provides the input optical signal 14s to the second erbium doped optical fiber 22 through the WDM coupler 32c. By the third optical isolator 13, unwanted lights, such as pumping lights in the backward direction are blocked from travelling toward the input side.
The L-band optical amplifier 200 works the same way as the C-band optical amplifier 100. The second erbium doped optical fiber 22 is configured to have a fiber length most suitable for amplifying L-band optical signals. For example, the second erbium doped optical fiber 22 has a fiber length of 120 m (meter). As noted above, the second pump light source 32 is provided between the third optical isolator 13 and the second erbium doped optical fiber 22. Further, the third pump light source 33 is provided between the fourth optical isolator 14 and the second erbium doped optical fiber 22. Under this configuration, an L-band light signal can be amplified by several ten dB, for example, 20 dB or more.
As noted above, in order for the second erbium doped optical fiber 22 to amplify the L-band light signal, the length of the erbium doped optical fiber must be relatively long, for example, 120 m. Since the second erbium doped optical fiber 22 is long, it requires bidirectional pumping or high power pump lights to excite the optical fiber. In the example of FIG. 1, the pump light sources 32 and 33 are provided both the front side and back side of the second erbium doped optical fiber 22 (bidirectional pumping).
The WDM coupler (optical multiplexer) 62 is used for combining two input lights, from the C-band and L-band optical amplifiers, respectively, and producing a combined optical signal at its output. Namely, the WDM coupler 62 receives the C-band optical signal 12s from the C-band optical amplifier 100 and the L-band optical signal 13s from the L-band optical amplifier 200 and outputs a combined optical signal 62s. Instead of the WDM coupler 62 noted above, other type of optical multiplexer or an optical switch may be used.
As described in the foregoing with reference to FIG. 1, in the wideband optical amplifier ranging from the C-band to L-band, the optical signals passing through the optical isolators 11 and 13, which limit the direction of the signals, are amplified by the erbium doped optical fibers 21 and 22 excited by the corresponding pump lights from the pump light sources 31, 32, and 33. The amplified optical signals are output through the corresponding optical isolators 12 and 14. In such an arrangement, it is known that the bandwidth or band of wavelengths of the optical amplifier can be controlled by varying the fiber length of the erbium doped optical fibers 21 and 22 and the intensity of the pump lights. For example, by increasing the fiber length of the erbium doped optical fibers, the wavelength of the signals to be amplified is increased.
As explained in the foregoing, in the conventional wideband optical amplifier of FIG. 1, for amplifying optical signals ranging from the C-band to L-band, several pump lights must be used. Further, the optical isolators are required at both the input side and the output side of each of the C-band and L-band amplifiers. Moreover, the optical demultiplexer and multiplexer are also necessary to divide and combine the light signals. Because the conventional optical amplifier requires many optical components, the amplifier involves a relatively large insertion loss as well as high cost. Moreover, the optical amplifier needs to have erbium doped optical fibers of considerable lengths. For example, as noted above, the optical amplifier includes both the first erbium doped optical fiber of 20 m for the C-band amplifier and the second erbium doped optical fiber of 120 m for the L-band amplifier.
It is, therefore, an object of the present invention to provide a wideband optical amplifier for amplifying an input optical signal of known wavelength in one of at least two bands of wavelength with a significantly small number of optical components.
It is another object of the present invention to provide a wideband optical amplifier for amplifying an input optical signal of known wavelength in one of at least two bands of wavelength with a simple structure and low cost.
It is a further object of the present invention to provide a wideband optical amplifier for amplifying an input optical signal of known wavelength in one of at least two bands of wavelength which has an improved signal-to-noise ratio while reducing cost and a number of components.
It is a further object of the present invention to provide a wideband variable wavelength optical source for generating an optical signal ranging at least two bands of wavelength with a simple structure and low cost.
To achieve the above object, the first aspect of the wideband optical amplifier of the present invention includes: a first set of a first optical coupler, a first pump light source, and a first erbium doped optical fiber for exciting the first erbium doped optical fiber by a first pump light from the first pump light source; an optical switch for changing paths for an output signal of the first set; and a second set of a second optical coupler, a second pump light source, and a second erbium doped optical fiber for exciting the second erbium doped optical fiber by a second pump light from the second pump light source.
The first set constitutes a first optical amplifier for a first band of amplification and a second optical amplifier for a second band of amplification by a combination of the first set and the second set constitutes. The first and second erbium doped optical fibers are adjusted in lengths and/or density of erbium doping to match the first and second bands of amplification.
The second aspect of the present invention is a wideband variable wavelength optical source utilizing the wideband optical amplifier noted above for generating an optical signal in one of at least two bands of wavelength. The wideband variable wavelength optical source includes: a first optical amplifier having a first optical coupler, a first pump light source, and a first erbium doped optical fiber for exciting the first erbium doped optical fiber by a first pump light from the first pump light source; an optical switch for changing paths for an output signal of the first optical amplifier; an amplifier block having a second optical coupler, a second pump light source, and a second erbium doped optical fiber for exciting the second erbium doped optical fiber by a second pump light from the second pump light source; a second optical amplifier formed by connecting the first optical amplifier and the amplifier block in series through the optical switch; a variable wavelength optical filter for selecting a wavelength of the optical signal to be generated by the wideband variable wavelength optical source; and an optical demultiplexer for forming a closed loop by returning the optical signal from the variable wavelength optical filter to an input of the first optical amplifier and producing the optical signal as an output.
The first and second erbium doped optical fibers are adjusted in lengths and/or density of erbium doping to match the first and second bands of amplification. Alternatively, a length of the first erbium doped optical fiber is adjusted to match the first band and a sum of lengths of the first erbium doped optical fiber and the second erbium doped optical fiber is adjusted to match the second band.
The third aspect of the present invention is a wideband optical amplifier having at least two bands of wavelength for amplifying an input optical signal of known wavelength and having an improved signal-to-noise (S/N) ratio. The wideband optical amplifier includes: a first optical amplifier for amplifying a first band optical signal and formed of a first optical coupler, a first pump light source, and a first erbium doped optical fiber for exciting the first erbium doped optical fiber by a first pump light from the first pump light source; an optical switch for changing paths for an output signal of the first optical amplifier; and a second optical amplifier for amplifying a second band optical signal which is longer in wavelength than that of the first band and formed of the first optical amplifier and a second amplifier block having a second optical coupler, a second pump light source, and a second erbium doped optical fiber for exciting the second erbium doped optical fiber by a second pump light from the second pump light source wherein the second optical amplifier includes means for removing an amplified spontaneous emission (ASE) light in the first band from the second erbium doped optical fiber.
The means for removing the amplified spontaneous emission (ASE) light in the first band is a wavelength selective optical coupler which couples the second pump light to the second erbium doped optical fiber and prevents the ASE light in the first band from passing therethrough. Alternatively, the means for removing the amplified spontaneous emission (ASE) light in the first band is an optical filter which prevents the ASE light in the first band from passing therethrough.
According to the present invention, the wideband optical amplifier can eliminate expensive optical components by a series connection of the first and second optical amplifiers. Thus, significant cost reduction as well as reduction in size can be achieved. Moreover, the fiber length of the second erbium doped optical fiber is decreased, and the power level of the pump light for pumping the second erbium doped optical fiber can be accordingly decreased, resulting in further reduction in size and cost. The wideband variable wavelength optical source using the wideband optical amplification can also achieve the same advantages noted above. Further, the wideband optical amplifier can improve the signal-to-noise (S/N) ratio in the L-band amplification by incorporating a filter function that blocks the amplified spontaneous emission (ASE) in the C-band wavelength.